The researchers examined 945 movies, counting the appearances of overall violence in each five-minute segment of a sampling of films that ranked among the top 30 at the domestic box office from 1950 to 2012. Perhaps not surprisingly, the results -- reported by Brad J. Bushman of Ohio State University, and Patrick E. Jamieson, Ilana Weitz and Daniel Romer of the Annenberg center -- showed that violence in American films had more than doubled in that time.

But the authors also found that episodes of gun violence in PG-13 rated films had been rising since the rating was introduced in the mid-1980s, and it now surpasses the violence in R-rated films, which are technically not open to young viewers unless they are accompanied by an adult.

The ratings system needs revising, concluded the authors of the study. One researcher also noted the MPAA's puritanical stance against sex compared to its lax standards for violence. "We treat sex as R," Romer said. "We should treat extreme gun violence as R." Amen.